Control circuit for varying the intensity of flash lamps

ABSTRACT

In an airport runway flashing light beacon system the intensity of emission of individual xenon flash lamps is varied by shunting capacitative discharge from the flash lamp to a lower impedance, non-emissive discharge tube at the end of selected intervals after initiation of the flash discharge. The intensity of flash lamp emission is dependent on the interval of its discharge. This interval is varied by a one cycle multivibrator time delay circuit responsive to the initiation of flash lamp discharge to produce a trigger pulse for the discharge tube after an interval selected by one of several switchable resistors or capacitors in the time delay circuit.

United States Patent [1 1 -Bonazoli et al.

[111 3,753,039 [451 Aug. 14, 1973 Ellison I'I. Klrkhufl, Needham, bothof Mass.

[73] Assignee: GTE Sylvanla Incorporated,

Danvers, Mass.

[22] Filed: Sept. 8, 1971 [2|] Appl. No.: 178,628

[56] References Cited UNITED STATES PATENTS 3,122,677 2/l964 Flieder315/241 R 3,519,984 7/1970 Zychal 340/25 Primary Examiner-Roy LakeAssistant Examiner-Lawrence J .Dahl Attorney-Norman J. O'Malley [57]ABSTRACT In an airport runway flashing light beacon system the intensityof emission of individual xenon flash lamps is varied by shuntingcapacitative discharge from the flash lamp to a lower impedance,non-emissive discharge tube at the end of selected intervals afterinitiation of the flash discharge. The intensity of flash lamp emissionis dependent on the interval of its discharge. This interval is variedby a one cycle multivibrator time delay circuit responsive to theinitiation of flash lamp discharge to produce a trigger pulse for thedischarge tube after an interval selected by one of several switchableresistors or capacitors in the time delay circuit.

INTENSITY CONTROL PATENIEnAuc'M Ian 3.753.039

sheen 0F 2 DII SI FT QT Fl MM Rl5 9 9 TI T3 7 T4 LL M kl c|2 u on me GJj INTENSITY CONTROL 270 ATTORNEY PATENIEDM 14 on 3.75.1039

sum 2 or 2 270 ,/'$3 Q2 QB 1 R22 7 L 2 O i 1% INVENTORS ROBERT P.BONAZOLI ELLISON H. KIRKHUFF BY anna/W ATTORNEY 1 CONTROL CIRCUIT FORVARYING THE INTENSITY F FLASH LAMPS BACKGROUND OF THE INVENTION Standardairport approach lighting installations include steady burning runwayedge lights and a series of flashing lamps or beacons aligned with therunway in advance of its threshold and operated successively so as toproduce the effect of a ball of light travelling at high speed towardthe runway. It has been customary to vary the intensity of the steadyburning lights according to the visibility, but to operate the flashlamps at full intensity in poor visibility and turn them off in goodvisibility to avoid blinding an approaching pilot. It is desired tocontrol flashing lamp intensity dependent on airport visibilityconditions, but because they are 'gas (xenon) discharge devices theirintensity is not easily controlled over the desired range from 100percent to 1 percent of full intensity.

Controlling intensity by varying the voltage applied to flash lamps islimited by lamp characteristics to a range of 2 to 1, yielding an outputcandlepower range of 5:1 far short of the desired 100:1 range. Inoperation each flash lamp discharge is supplied from a high voltage(typically 2,000 volt) storage capaciter. Capacitors of different valuesmight be switched to provide different stored charges, but would requirebulky and expensive high current, high voltage switching devices subjectto failure and welding of contacts. In photography it is also known tointerrupt flash lamp discharge by shunting the storage capacitor currentthrough a low impedance discharge tube so as to shorten the naturaldischarge time of the flash lamp storage capacitor and reduce theeffective intensity of eachflash. U.S. Pat. Nos. 3,033,988 and3,350,603, for example, disclose a flash lamp printer circuit and acamera flash exposure circuit respectively, in which the emission of theflash lamp is photoelectrically measured and used to discharge ashunting quench tube and terminate lamp discharge. Such photographicflash control circuits involving photoelectric monitoring and feed backrequire an added photoelectric component. For example, whereas a pilotdesires a low flash intensity at night, photoelectric feed back ofreduced ambient light would tend to increase the flash intensity. On theother hand, while good visibility conditions might be sensedphotoelectrically on the ground tending to reduce flash intensity, apilot may be approaching the runway in fog and would require high flashintensity. In these examples dependence on photoelectric feedbackinterferes with the desired control of flash intensity. Becausephotoelectric flash circuits are dependent on the instantaneous flashlamp output, they are not adapted to control stepwise variation of theflash lamp between discrete, predetermined intensity levels.

Accordingly, the object of the present invention is to provide a controlcircuit for varying the intensity of a flash lamp substantially over thefull range of lamp intensities in predetermined steps, which isindependent on the instantaneous lamp output, and ambient lightconditions, which avoids the use of photoelectric components, obviatesthe need for high current, high voltage switching devices, and which iscompact and economical.

SUMMARY OF THE INVENTION According to the invention an electroniccircuit for controlling the discharge of stored current through a first,light emitting, discharge device and a-second discharge device in shunttherewith comprises a time delay circuit having an input for connectionto the first device and responsive to discharge therethrough to producea trigger signal a predetermined interval after initiation of discharge,said delay circuit including a plurality of impedances selectivelyconnectable in the delay circuit to vary the length of said intervalstepwise, and means for applying the trigger signal to the second deviceand cause discharge thereof thereby to terminate discharge of the lightemitting device, whereby the effective intensity of emission from thefirst device can be varied over substantially the full range of itsintensities independently of its instantaneous emission.

DRAWING DESCRIPTION General The dot-dash lines of FIG. 1 represent aprinted circuit board PCB about 2 X 2% X 3%inches in dimension andweighing 2 to 3 ounces. Within the dot-dash lines is a preferred form ofcontrol circuit for varying the effective intensity of a flash lamp ortube FT shown in a high voltage circuit appearing above the PCB. Thehigh voltage circuit, PCB,and relays K3 and K4 are installed in ahousing which is one of a number of housings, e.g. eight, located in aseries approaching anairport runway threshold. The series of flash larnpinstallations is under the control of a timer T which sequentiallyinitiates the discharge of successively approaching lamps. The sequencerepeats typically twice a second with an approximately 35 millisecondinterval between flashes of successive lamps. An intensity control I,located near the threshold of the runway senses the current carried bysteady burning runway edge lights, which current is remotely controlledfrom an airport control facility. As the edge light current is steppedbetween high, medium and low intensities, the intensity control I sensesthe steps and energizes the relays K3 and K4 so as to step the flashlamp FT through corresponding intensities.

FIG. 1 High Voltage Flash Lamp Circuit L1 to the primary of a powertransformer T1. The

2,700 volts at the center tap of the power transformer secondary isrectified by two 250 :milliampere, 3 kilovolt diodes D11 to provide2,000 volt direct current between the high voltage positive bus andground has G for charging a 30 microfarad main storage capacitor C11.The flash lamp FI and a quenchtube QT are connected in shunt across thestorage capacitor and are non-conductive until triggered by a voltagesignal applied to their respective controls c. The flash tube FT may be.a xenon filled gas discharge device, Sylvania type R4335, with animpedance on discharge of 22 ohms and a light emission of 14,000 lumenseconds. The quench tube may be a 12 mm. OD Pyrex tube, with an arclength of 12 inches and a fill of argon at ton. The impedance of thequench tube is preferably about one tenth that of the flash tube.Resistors R15 (1 megohm) and R16 (l20 kilohms) divide the voltage toapproximately 200 volts at their junction charging a secondary storagecapacitor C12 to this voltage. When the timer T momentarily energizes arelay Kl, its contact k1 allows the secondary capacitor C12 to dischargethrough an autotransformer T3 which steps up the ensuing voltage pulseto about 15 kilovolts as required to trigger the flash tube FT andinitiate its discharge and light emission. Simultaneously the dividerjunction voltage, which is connected through the PCB terminal S to thebase b of a transistor Q1, drops to the low ground voltage of Bus G.

FIG. 1 Printed Circuit Board PCB The PCB is supplied from the 240 voltpower terminals by a transformer T2 which steps down the 240 volt highvoltage circuit power to 120 volts. The lower voltage is rectified by abridge comprising diodes D1 to D4 inclusive and applied between a lowvoltage positive bus and a ground bus G commonly connected through PCBterminal K with the high voltage circuit ground. The positive bus isshown connected through a 15 kilohm resistor R3, PCB terminals B and R,and a relay contact k3 to a like positive bus of a PCB time delaysub-circuit TD regulated by a type 2VR20 zener diode D5.

Transistors Q1 and 02 (type 2N2222) of the PCB time delay sub-cirucit TDcomprise a one cycle multivibrator or monostable flip flop. The firststage 01 is normally held conducting by the positive voltage appliedthrough input terminal S further divided by resistors R4 lmegohm) and R5(100 kilohms). The second stage 01 is normally held out off byconnection of its base 12 through a 470 ohm resistor R8 to the collectorc of the first stage. When, at the initiation of the flash tubedischarge the voltage at the PCB input S and at the base b of Q1 dropsto ground, Q1 ceases to conduct and its collector voltage rises at anexponential rate determined by a time constant network comprising a 270ohm resistor R8 and one of three capacitors C3 (0.001 microfarad), C4(0.005 mf) or C5 (0.0033 mf) as selected by the intensity control relaysK3 and K4 (described more fully hereafter). Concomitantly the secondstage transistor 02 starts conducting at a rate determined by resistorR9 (220 ohms) and capacitor C6 (0.047 mfd). Conduction of the secondstage transistor 02 continues until capacitor C6 is charged to theemitting switching voltage of a unijunction transistor (UJT) Q3 (type2N4870). The time interval for reaching the UJT switching voltage isdetermined by the multivibrator time delay network and varies accordingto which of the three capacitors C3, C4 or C5 is selected. When the UJTis switched on, a sharp voltage pulse at its base bl is transmittedthrough an 0.1 mf capacitor C7 to the gate g of a silicon controlledrectifier (SCR) 04 (type 2N44 43). The SCR O4 is turned on abruptly anddischarges at a 2 mt trigger capacitor C2 through the primary of thequench tube autotransformer T4. A high voltage pulse in the T4 secondarytriggers the quench tube into conduction, thereby discharging the mainhigh voltage storage capacitor C1 and abruptly extinguishing the flashlamp FT. This cycle for each lamp may be repeated at half secondintervals.

FIG. l-Intensity Control-Relays K3 and K4 As previously suggested theeffective emission of the flash lamps is dependent upon the length ofthe interval between the initiation and quenching of flash lampdischarge, this interval being varied by selection of one of thecapacitors C3, C4 or C5 in the multivibrator time delay network by therelays K3 and K4.

As an example, it is desired to vary the effective flash intensitystepwise from a high level of percent of maximum intensity through amedium level of 26 or 8 percent, to a low level of 1 percent.

For the high level of flash intensity relay K3 is energized by theintensity control I transferring its contact k1 from the position shownin FIG. 1 to position 100 which disconnects the power supply from thepositive bus of the time delay circuit TD and disables the circuit TD.Consequently the quench tube QT will not be triggered and the flash tubeFT will discharge the full charge of the main capacitor C11 and emitfull intensity in about milliseconds.

The intermediate intensity of 26 or 8 percent is initially selected by amanual switch S1, shown in FIG. 1 in the 26 percent position. With relayK3 de-energized and relay K4 energized the contact k4 is transferredfrom the position shown in FIG. 1 to the position connecting capacitorC4 to the base of transistor Q2, and providing a time delay ofapproximately 22 microseconds between initiation and quenching of flashlamp discharge. If switch S1 is in the 8 percent position, capacitor C5is selected providing a time delay of approximately 16 microseconds.

For the lowest intensity level, 1 percent of maximum intensity, theintensity control I leaves both relays de- -energized and their contactsas shown in FIG. 1 thus selecting capacitor C3 which provides a timedelay of approximately 8 microseconds.

Thus in FIG. 1 the variably selected impedance of the time delay circuitTD which govern the flash lamp intensity are capacitors C3, C4 and C5.While stepwise charges of intensity of 1-8 percent or 26 percent aregiven as examples, the capacitors C3, C4, C5 may be of values providingsteps from 1 to 100 percent of maximum intensity.

FIG. 2 Alternate Control Circuit In FIG. 2 the variably selectedimpedances are resistors R19, R22 and R23 respectively 15, 9.1 and 22kilohms. The time delay circuit of FIG. 2 is like that of FIG. 1 and hasthe same components except as noted below.

Transistors Q1 and Q2 comprise a monostable multivibrator which executesone cycle when the voltage applied through terminal S drops sharply atthe initiation of flash lamp discharge. The resulting rising voltage atthe emitter e of the second stage 02 charges a capacitor C11 (0.047 mf)until the threshold voltage of a programmable unijunction transistor(PUT, General Electric type D13T) is reached. The threshold voltage isdetermined by the voltage dividers R19, R22 and R23. A selected voltageis applied through a diode D12 (lN4004) to the gate of the PUT Q13. Thisvoltage is selected by operation of relay K4 by the intensity control Iof FIG. 1. The values of the voltage divider resistor R19, R22 and R23are selected such that the PUT threshold is reached, for example, afterthe time intervals described with respect to FIG. 1. After such timeintervals the PUT conducts generating a trigger pulse which is appliedthrough a capacitor C2 at the output of SCR Q4 to the triggertransformer T4 of the quench tube QT control c. A diode D13 (1N4004)permits rapid charging of the capacitor during transmission of thetrigger pulse to the quench tube. Discharge of the quench tube QTterminates discharge of the flash tube FT when its intensity has reachedthe level of l or 8 percent of maximum preselected according to thecontact positions of relay K4. For 100 percent intensity a relay K3,operated by the intensity control I as in FIG. 1, transfers its contactk3 to the 100 percent, disconnecting the positive bus from the timedelay circuit and preventing flashing of the quench tube QT so that theflash tube FT discharges the full charge of the main capacitor C11.

With the two exemplary forms of the invention it can be seen that thesmall and inexpensive components of the low voltage PCB time delaycircuit are distinctly advantageous over high voltage, high currentcomponents in the flash tube circuit, and over photoelectric controlswhich depend on the flash lamp light output or light feedback. Whereasphotoelectric circuits are notoriously subject to drift, the selectablecapacitative and resistive impedances will vary only one or two percentover long periods (years) of time.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents falling within the scope of the appended claims.

We claim:

1. An electronic circuit for controlling the discharge of stored currentthrough a first, light emitting, discharge device and a second dischargedevice in shunt therewith comprising:

a time delay circuit having an input for connection to the first deviceand responsive to discharge therethrough to produce a trigger signal apredetermined interval after initiation of discharge, said delay circuitincluding a plurality of impedances selectively connectable in the delaycircuit to vary the length of said interval stepwise,

output means for applying the trigger signal to the seconddevice andcause discharge thereof thereby to terminate discharge of the lightemitting device, and

means connected to the impedances for switching selected impedances insaid circuit,

whereby the effective intensity of emission from the first device can bevaried in predetermined values over substantially the full range of itsintensities independently of its instantaneous emission.

2. A circuit according to claim 1 wherein said time delay circuitcomprises a switching device having a control connected to said input,and responsive to a voltage change at said input to initiate currentflow through a selected impedance.

3. A circuit according to claim 2 wherein said switching devicecomprises the first stage of a one cycle multivibrator with a changingvoltage output.

4. A circuit according to claim 3 wherein said multivibrator includes atime constant network comprising said impedances.

5. A circuit according to claim 1 wherein said time delay circuitcomprises a trigger device with a control causing transmission of thetrigger signal when the voltage at said control crosses a thresholdvalue at the trigger control, said impedances being coupled to saidcontrol to select said threshold value.

6. A circuit according to claim 2 wherein said time delay circuitcomprises a trigger device connected to said multivibrator and having acontrol causing transmission of the trigger signal when themultivibrator voltage output crosses a threshold value at the triggercontrol.

7. A circuit according to claim 6 wherein said multivibrator includes atime constant network comprising said impedances.

8. A circuit according to claim 6 wherein said imped ances are coupledto said control.

9. A circuit according to claim 1 wherein said impedances comprisecapacitances.

10. A circuit according to claim 3 wherein said impedances comprisecapacitances.

11. A circuit according to claim 1 wherein said impedances compriseresistances.

12. A circuit according to claim 5 wherein said impedances compriseresistances.

13. A circuit according to claim 1 wherein said light emitting dischargedevice comprises an airport beacon flash lamp.

14. An airport beacon flash lamp system comprising:

a high voltage main storage capacitor;

a gas discharge flash lamp and quench tube connected to the storagecapacitor in a discharge circuit, each having a control for initiatingdischarge from the capacitor, discharge of the quench tube terminatindischarge of the flash lamp and limiting the effective intensity of theflash;

a low voltage, variable time delay circuit having an input connected tothe discharge circuit, a plurality of impedances selectively connectablein the time delay circuit, a switching device having a control connectedto the input and responsive to initiation of flash lamp discharge toinitiate current flow through and a changing voltage at the selectedimpedance, a trigger device with a control coupled to the selectedimpedance and having a threshold response to the changing voltage toproduce a trigger signal an interval after initiation of flash lampdischarge predetermined by the selected impedance;

means for applying the trigger signal to the quench tube control todischarge the quench tube and terminate flash lamp discharge; and

an intensity control including relay means for connecting selectedimpedances in the time delay circuit thereby to vary the intervalbetween flash lamp and quench tube discharge stepwise in predeterminedlengths,

whereby the effective intensity of flash lamp can be variedsubstantially over the full range of its intensities independently ofits instantaneous emission.

15. A system according to claim 14 wherein said impedances arecapacitative.

16. A system according to claim l4 wherein said impedances areresistive.

17. A system according to claim 14 wherein said intensity controlincludes addition relay means for disabling the time delay circuit fromapplying a trigger signal to the quench tube, thereby allowing the flashtube to emit full intensity.

i l i

1. An electronic circuit for controlling the discharge of stored currentthrough a first, light emitting, discharge device and a second dischargedevice in shunt therewith comprising: a time delay circuit having aninput for connection to the first device and responsive to dischargetherethrough to produce a trigger signal a predetermined interval afterinitiation of discharge, said delay circuit including a plurality ofimpedances selectively connectable in the delay circuit to vary thelength of said interval stepwise, output means for applying the triggersignal to the second device and cause discharge thereof thereby toterminate discharge of the light emitting device, and means connected tothe impedances for switching selected impedances in said circuit,whereby the effective intensity of emission from the first device can bevaried in predetermined values over substantially the full range of itsintensities independently of its instantaneous emission.
 2. A circuitaccording to claim 1 wherein said time delay circuit comprises aswitching device having a control connected to said input, andresponsive to a voltage change at said input to initiate current flowthrough a selected impedance.
 3. A circuit according to claim 2 whereinsaid switching device comprises the first stage of a one cyclemultivibrator with a changing voltage output.
 4. A circuit according toclaim 3 wherein said multivibrator includes a time constant networkcomprising said impedances.
 5. A circuit according to claim 1 whereinsaid time delay circuit comprises a trigger device with a controlcausing transmission of the trigger signal when the voltage at saidcontrol crosses a threshold value at the trigger control, saidimpedances being coupled to said control to select said threshold value.6. A circuit according to claim 2 wherein said time delay circuitcomprises a trigger device connected to said multivibrator and having acontrol causing transmission of the trigger signal when themultivibrator voltage output crosses a threshold value at the triggercontrol.
 7. A circuit according to claim 6 wherein said multivibratorincludes a time constant network comprising said impedances.
 8. Acircuit according to claim 6 wherein said impedances are coupled to saidcontrol.
 9. A circuit according to claim 1 wherein said impedancescomprise capacitances.
 10. A circuit according to claim 3 wherein saidimpedances comprise capacitances.
 11. A circuit according to claim 1wherein said impedances comprise resistances.
 12. A circuit according toclaim 5 wherein said impedances comprise resistances.
 13. A circuitaccording to claim 1 wherein said light emitting discharge devicecomprises an airport beacon flash lamp.
 14. An airport beacon flash lampsystem comprising: a high voltage main storage capacitor; a gasdischarge flash lamp and quench tube connected to the storage capacitorin a discharge circuit, each having a control for initiating dischargefrom the capacitor, discharge of the quench tube terminatin discharge ofthe flash lamp and limiting the effective intensity of tHe flash; a lowvoltage, variable time delay circuit having an input connected to thedischarge circuit, a plurality of impedances selectively connectable inthe time delay circuit, a switching device having a control connected tothe input and responsive to initiation of flash lamp discharge toinitiate current flow through and a changing voltage at the selectedimpedance, a trigger device with a control coupled to the selectedimpedance and having a threshold response to the changing voltage toproduce a trigger signal an interval after initiation of flash lampdischarge predetermined by the selected impedance; means for applyingthe trigger signal to the quench tube control to discharge the quenchtube and terminate flash lamp discharge; and an intensity controlincluding relay means for connecting selected impedances in the timedelay circuit thereby to vary the interval between flash lamp and quenchtube discharge stepwise in predetermined lengths, whereby the effectiveintensity of flash lamp can be varied substantially over the full rangeof its intensities independently of its instantaneous emission.
 15. Asystem according to claim 14 wherein said impedances are capacitative.16. A system according to claim 14 wherein said impedances areresistive.
 17. A system according to claim 14 wherein said intensitycontrol includes addition relay means for disabling the time delaycircuit from applying a trigger signal to the quench tube, therebyallowing the flash tube to emit full intensity.